Browsing by Author "Anane, Timothy"

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    Reconstructing late Holocene solar-driven surface UV-B variability using pollen UV-B absorbing compounds (UACs) : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Earth Science at Massey University, Palmerston North, New Zealand. EMBARGOED until 20/10/2026
    (Massey University, 2025-10-15) Anane, Timothy
    Stratospheric ozone, Earth’s primary shield from biologically active UV-B radiation, has seen significant depletion since the late 20th century due to anthropogenic emissions of chlorofluorocarbons (CFCs) and other ozone depleting substances (ODSs). The Montreal Protocol has resulted in a gradual decline of these substances in the atmosphere, and recovery is expected over the coming decades. However, recent reports of record UV-B levels in the Arctic and unusually large and persistent ozone holes over Antarctica, partly linked to events such as the 2019–2020 Australian wildfires, point to the continued fragility of the ozone layer. These developments reinforce the need for a better understanding of historical, pre-industrial surface UV-B levels, in order to identify key driving mechanisms and place recent trends in context. While ground-based spectroradiometers and instruments aboard satellites have enabled systematic UV-B monitoring in the space age, these records span only a few decades, with long-term reliability affected by instrument degradation and calibration drift. This study explored a novel surface UV-B proxy that is based on the upregulation of secondary metabolites that act as UV-B absorbing compounds (UACs, e.g. ferulic and para-coumaric acids), when plants are subjected to elevated UV-B conditions. Preservation of these compounds in ancient pollen grains is enabled by the chemically robust outer wall or exine, which is composed of the biopolymer sporopollenin. Although the proxy has been used to reconstruct surface UV-B records capturing variations on timescales ranging from multidecadal to multimillennial, its capacity to resolve higher frequency variability, such as across the 11-year solar cycle and short-lived perturbations in ozone associated with solar energetic particle (SEP) events remain untested. Fourier transform infrared (FTIR) microspectroscopy was employed to semi-quantify UACs recovered from historical and sub-fossil Pinus pollen grains obtained from annually varved sediments of Nar Gölü, a volcanic crater lake in central Türkiye, across four different intervals: a biennially resolved record spanning the satellite era (1983–2010), a second biennial record covering the Maunder minimum (1635–1714 CE), an annually resolved reconstruction covering the 774/775 CE SEP event (764–783 CE), and a quasi-centennially resolved record spanning 2000–300 BCE and 500–1900 CE. The biennially resolved UAC records of the satellite era and Maunder minimum reveal quasi-decadal cycles of ~11.4 and ~12 years respectively, consistent with the solar cycle. For the satellite era, a statistically significant positive correlation is observed between UAC-inferred surface UV-B and solar activity, including solar UV-B and Total Solar Irradiance (TSI). For the Maunder minimum, maxima, and minima in UAC associated with cycles ranging from 8–30 years correspond to those in multiple solar activity proxies: solar modulation potential, 10Be inferred solar cycles, reconstructed sunspot group number and TSI. The annually resolved record also exhibits a quasi-decadal variability, characterised by a peak-to-peak duration of ~12.5 years and a trough-to-trough duration of ~10.5 years. Aside the sinusoidal pattern, an abrupt spike in UAC is observed between 776 and 777 CE, with increases of 66% and 52% above background averages, in the first and second years, respectively. This anomaly aligns with the timing of the 774/775 CE SEP event in Δ¹⁴C and ¹⁰Be records from tree rings and Greenland and Antarctic ice cores. The magnitude and timing of the increase suggests a short-lived surface UV-B enhancement driven by NOx initiated ozone depletion following the SEP event. Additionally, the occurrence of this anomaly within an inferred solar minimum supports recent evidence that the SEP event likely occurred during solar minimum rather the solar maximum. The quasi-centennial late Holocene record reveals modulation of surface UV-B by solar activity, with additional influence from orbital forcing. Solar influence on the earlier interval (2000–300 BCE) appears stronger than in the later one, as indicated by a significant positive correlation between UAC abundance and TSI, in contrast to the weaker, non-significant positive relationship observed from 500–1900 CE. Additionally, surface UV-B exhibits a gradual increase from the late Holocene into the common era, aligning with the rise in spring insolation at 38°N over the same period. These findings support the utility of sporomorph UACs as a proxy for reconstructing surface UV-B flux across a range of timescales, capturing both periodic solar cycles and abrupt short-lived events, as well as longer-term variability influenced by solar and orbital forcing. By extending direct UV-B reconstructions into the pre-industrial past, this work establishes a new foundation for understanding natural solar–ozone interactions and constraining the limits of surface UV-B variability under changing solar and atmospheric conditions.